Typically, the rotor has a claw-type structure. A structure of this type is constituted by two opposite, imbricated polar wheels, each comprising claws. Each claw of a polar wheel extends in the direction of the other polar wheel. In addition, each claw is inserted between two consecutive claws of the opposite polar wheel. Additionally, the rotor comprises magnetic parts, and the excitation coil is wound around the rotation shaft of the machine.
The rotor is located inside a stator. When the rotary machine is functioning, the rotor rotates around its axis, and a magnetic flow circulates between the adjacent magnetic poles with respective opposite polarities, by passing via the windings of the stator. The magnets which are disposed between the magnetic poles, constituted by the claws, are used to prevent the magnetic flow from passing directly from one pole to the other, without passing via the stator.
In fact, this leakage of flow from one pole to the other, without passing via the stator, affects the output, and detracts from the power performance of the rotary electrical machine. In fact, the flow which passes directly from one claw to the other without passing via the stator does not participate in the functioning of the machine. The use of interpolar magnets makes it possible to limit these leakages.
Interpolar magnets with a globally parallelepiped form are known. Each is placed between two adjacent claws of the rotor, each belonging respectively to one of the two polar wheels. These interpolar magnets are retained either by hooks, or by means of two grooves (or shoulders), each provided in one of the opposite lateral edges of the claws between which the magnet is situated.
In the first case, the magnets can have a reduced size, but they tend to move because of centrifugal force, since the hooks do not guarantee optimum securing.
In both cases, the magnets must have a volume which is sufficient to fill the entire interpolar gap, and they can be machined to have a rib which is designed to co-operate with the groove in the polar claws. This increases the cost of the machine, since these magnets are expensive.
In applications in which the rotary electrical machine must provide a high power density, the use of a large number of these magnets is essential.
Because of the cost of the raw material from which they are made, for example rare earths or ferrite, these interpolar magnets represent a substantial part of the cost of the rotor. Consequently, their design needs to be improved. In particular, it is necessary to optimise their geometric form in order to ensure that all of the mass of the magnet is useful for its functions, whilst guaranteeing good mechanical strength.